The present invention relates to a non-volatile memory device and a method of operating the same, in which a program voltage of the non-volatile memory device is controlled variably according to the number of a program/erase operation.
In recent years, there is an increasing demand for non-volatile memory devices which can be electrically programmed and erased and do not need the refresh function of rewriting data at certain intervals.
The non-volatile memory device is configured to perform the program or erase operation by injecting electrons or deleting electrons into or from the floating gate of each cell.
A NAND type non-volatile memory device of the non-volatile memory devices includes a number of cell strings in which a plurality of memory cells is connected in series. In terms of the cell string structure, when a specific cell is read, unselected cells other than a selected cell are bypassed or turned on. Thus, to the word line of the unselected cell is applied a pass voltage higher than the threshold voltage of a corresponding cell in order to turn on the corresponding cell. However, unselected cells, which are programmed using a threshold voltage higher than that of other unselected cells because they have a faster program speed, may not be turned on although a corresponding pass voltage is applied. If there exist cells that are not turned on as described above, it is very difficult to precisely determine whether a selected cell has been programmed in a corresponding cell string.
This is because cells whose threshold voltage rises rapidly and cells whose threshold voltage rises relatively slowly coexist even though the same program voltage is applied to both the cells. In order to prevent this problem, the incremental step pulse program (ISPP) programming method of performing programming on cells having a fast program speed at a relatively low program voltage and on cells having a slow program speed at a relatively high program voltage.
However, the characteristics of a cell or the program speed of a cell is always not uniform. If the program and erase operations are executed repeatedly, the tunnel oxide layer of a cell is damaged and/or traps, etc., are generated. Consequently, the program speed rises on the whole.
As described above, as the program and erase operations are repeated, the program speed of a cell rises. Accordingly, there is a need to control a program voltage.